Which of the following is a non-sense codon?

Question

UnlockIAS · Accepted Answer

Correct — A, UAA. A codon is a triplet of bases on messenger RNA, and of the 64 possible triplets, 61 specify an amino acid — these are the sense codons — while exactly three specify none at all. Those three are UAA, UAG and UGA, and they are called nonsense codons precisely because they make no sense to a transfer RNA: no tRNA carries an anticodon for them. When the ribosome arrives at one, nothing enters the A site except a release factor protein, which hydrolyses the bond holding the finished polypeptide to the last tRNA and lets the chain go. So the nonsense codons are also called stop, termination or chain-terminating codons, and each has a nickname from the early laboratory work — UAA is ochre, UAG is amber and UGA is opal or umber. UAA is the one printed in this question, so option (a) is the answer; the nicknames are not decoration but laboratory history, since it was Sydney Brenner's mutation experiments that established that the amber and ochre mutations corresponded to the triplets UAG and UAA. The other three options are all sense codons, and each is famous for a different reason, which is exactly what makes this a well-built four-option item rather than a lucky guess: AUG is the start codon and codes methionine, UUU codes phenylalanine and was the very first codon ever deciphered, and UGC codes cysteine. Notice the built-in tripwire — AUG and UAA are made of overlapping letters and are the two functionally opposite codons on the paper, initiation against termination. The standard memory aid keeps them apart by reading the stop codons as sentences: U Are Away (UAA), U Are Gone (UAG), U Go Away (UGA). All three begin with U, and no start or sense codon in the trio does. A useful cross-check while revising: a nonsense mutation is a point change that turns a sense codon into one of these three, truncating the protein — beta-thalassaemia and some Duchenne muscular dystrophy alleles arise this way — which is where the word 'nonsense' in the question stem comes from.

Why the other options are wrong — (b) The exact opposite of a nonsense codon. AUG is the initiation or start codon: it codes methionine in eukaryotes and formyl-methionine in bacteria, and every polypeptide begins its synthesis there, the methionine often being clipped off afterwards. AUG is the only codon that does double duty, acting as both a start signal and an ordinary methionine codon in the middle of a message. Candidates pick it because its three letters overlap with UAA and UGA, and because it is the one codon everyone has memorised.  (c) UUU codes phenylalanine, and it is the most historically important codon of all: in 1961 Marshall Nirenberg and Heinrich Matthaei fed a synthetic poly-U messenger to a cell-free extract of E. coli and obtained a polypeptide made entirely of phenylalanine, cracking the first letter-to-amino-acid assignment in the genetic code. It is a sense codon in every organism. Its run of three identical bases makes it look artificial or 'blank' to a candidate skimming the options, which is the whole reason it is here.  (d) UGC codes cysteine, the sulphur-containing amino acid whose thiol groups form the disulphide bridges that hold protein tertiary structure together — including those that link the two chains of insulin. It is an ordinary sense codon. The trap is visual: UGC is one letter away from UGA, which IS a stop codon, so a candidate who has memorised the shapes rather than the sequences can read UGA where UGC is printed.

Concept: The genetic code is the dictionary that converts a nucleotide sequence into an amino acid sequence. Because there are four bases (A, U, G, C in RNA) and twenty amino acids to specify, a one-base code would name only four amino acids and a two-base code only sixteen; a triplet gives 4 x 4 x 4 = 64 combinations, which is enough and to spare. That surplus produces the two defining properties of the code. It is DEGENERATE — most amino acids are named by more than one codon, leucine, serine and arginine by six each — but it is not ambiguous, since any one codon specifies only one amino acid. It is also non-overlapping and comma-less, read three bases at a time from a fixed starting point, which is why inserting or deleting a single base causes a frameshift that garbles everything downstream. Of the 64 codons, 61 are sense and 3 are nonsense. The code is very nearly universal: the same triplets mean the same amino acids in a bacterium, a mango tree and a human being, which is the reason a human insulin gene can be expressed in E. coli at all. The decoding was done between 1961 and 1966, and Indian students should know the Indian-born name in it — Har Gobind Khorana shared the 1968 Nobel Prize in Physiology or Medicine with Marshall Nirenberg and Robert Holley for the interpretation of the genetic code and its function in protein synthesis.

State PCS 2025 Q143 — Science & Technology PYQ with Answer

State PCS 2025 Q143 — Science & Technology PYQ with Answer